1. Technical Field
Present invention generally relates to methods for transferring data between multiple independent stations. More particularly, the present invention relates to data transfer in a local area network between a number of data processing devices--such as personal computers, lap top computers, or palm top computers--where the devices use a wireless media--such as infrared, radio frequency or microwaves--or interconnecting cable, if desired.
2. Description of the Related Art
There are a number of techniques for communicating between independent data processing stations using well known medium such as infrared or radio frequency.
In an article entitled "A New Channel Access Message For Packet Radio," which was published in the proceedings of the ARRL Ninth Computer Networking Conference, London, Ontario, Canada, Sep. 22, 1990, ISDN0-87259-337-1 by Phil Karn, there is disclosed a channel access method for use by multiple packet radio stations on a shared simplex packet radio channel which permits different stations to initiate independent access to the channels and initiate conversations with other stations. The article proposes a technique for multiple access with collision avoidance to reduce the interference of stations with each other. However, there is no disclosure of any message for providing fairness of access to maximize the utilization of the medium by each station and reduce the waiting of each station for access to the medium; and there is no mechanism for making medium reservations between stations, nor is there a mechanism for terminating conversations.
U.S. Pat. No. 5,231,634, issued Jul. 27, 1993 and assigned to Proxim, Inc., discloses a medium access protocol for wireless LAN which requires a station intending to initiate communications to send reservation duration information in its request to the intended receiving station to reserve a fixed block of time for its proposed communications or conversation. This may pose some problems as there is no allowance provided for retries, and if there is a problem in communications, a new reservation is required which may necessitate delays because other stations may have initiated communications at the end of the reservation.
U.S. Pat. No. 4,661,902, issued Apr. 28, 1987 and assigned to Apple Computer, Inc., discloses a local area network with carrier sense collision avoidance. However, it does not disclose a means by which the stations not involved in the conversation acknowledge and obey a conversation reservation. The system disclosed is a hardwired system which does not address the problem of "hidden terminals," and can therefore depend wholly upon Carrier Sense Multiple Access With Collision Avoidance (CSMNCA). This reference shows the initiation of conversations by the transmission of a request to send message by the transmitting station and a clear to send message sent back to the transmitting station by the intended receiving station. The non-conversant stations perform a carrier sense to detect when the communication medium is free for their use. Thus, they are free to try to initiate conversations during the conversations of other stations.
Unlike a wired environment, in a wireless environment-such as infrared or radio frequency--not all stations are able to hear all the other stations. This is referred to as the "hidden terminal" problem. The problem with hidden terminals is that they need to be made aware of the reservation effected by a communicating pair of stations so that they do not interrupt the conversation and cause a collision. This problem occurs in wireless local area networks (LANs), as it is possible for one or more station (or terminal) to be intermittently shielded from the others--for instance, by being moved or by the interposition of an obstruction. This is particularly important in a portable environment, as a portable station, such as a lap top or palm top, are quite mobile and may move out of range.